The invention relates to a method for preparing a slow/controlled release fertilizer by use of a modified urea resin coating material. The method comprises the following steps: step 1, synthesizing modified urea resin; step 2, preparing a curing agent solution, wherein a curing agent is formed by compounding two or more than two of ammonium chloride, ammonium sulfate, urea and ammonia water, and the curing agent is added with water to prepare and form the curing agent solution; step 3, preparing the slow/controlled release fertilizer. The method has the benefits that the nutrient content of the fertilizer is increased, a membrane causes no secondary pollution to the environment and plants, a membrane coating is high in strength and excellent in wear resistance, the crop yield can be increased and the utilization ratio of the fertilizer efficiency is improved.